Separation discs of metal are used in connection with relatively robust and large-sized centrifugal separators for separating liquid mixtures; wherein the separation discs are of relatively large size and exposed to both high centrifugal and liquid forces.
The separation discs are stacked at a mutual distance to form interspaces between themselves. The liquid mixture to be separated in the centrifugal rotor is conducted through the interspaces, wherein the liquid mixture is separated into phases of different densities during operation of the centrifugal separator. The interspaces are provided by said spacing members arranged on the surface of each separation disc. There are many ways of forming such spacing members. They may be formed by attaching separate members in the form of narrow strips or small circles of sheet metal to the separation disc, usually by spot welding them to the surface of the separation disc. The separation disc may also be manufactured with spacing members formed in one piece with the material of the separation disc itself. Known techniques for manufacturing separation discs with integrally formed spacing members are disclosed in for instance WO 2007/055630 A1 and U.S. Pat. No. 6,526,794 B1.
The size of the interspaces between the separation discs depends on how much the spacing members extend or protrude from the surface, i.e. the height of the spacing members. Dimensioning the size of the interspaces or height of the spacing members involves different aspects to be considered. For instance, it will depend on the type and amount of solids (sludge) being suspended in the liquid mixture. In general, the height (or size of the interspaces) will be dimensioned somewhere in the range of 0.3 to 0.8 mm.
Furthermore, the centrifugal rotor provides a separation space which is designed with a given total height for the stack of separation discs. In order to maximize the separating capacity of the centrifugal separator, there is a de-sire to fit as many separation discs as possible into the stack within that given height. More separation discs in the stack means more interspaces in which the liquid mixture can be separated. An optimum height on each of the individual interspaces will however usually be given by (or depend on) the type of liquid mixture which is intended to be separated. Consequently, this leaves the option of making the separation discs as thin as possible to maximize the number of separation discs within said given total stack height.
However, there's a lower limit as to how thin the separation discs can be made. Present day manufacturing techniques and in particular the material of the separation disc will define this lower limit. The thickness of the separation discs (i.e. without counting the spacing members) will typically be somewhere in the range from 0.3 to 0.6 mm. A separation disc having a small size (diameter) may exhibit a thickness of 0.4 mm, whereas a substantially larger separation disc may exhibit a thickness as great as 0.7 mm. Hence, the separation disc is in general made thicker with the size (diameter) of the separation disc. The centrifugal rotor will rotate at high speeds, and accordingly the separation discs are exposed to high centrifugal forces and strains during rotation. If the separation discs are made too thin, such strains would lead to material failure or permanent deformation.
Prior to this happening, there may be other problems with very thin separation discs. As the separation discs are made thinner, they will exhibit a loss in rigidity and irregularities in their shape may begin to appear. The separation discs are furthermore compressed in the stack inside the centrifugal rotor to form a tight unit. The thin separation discs may thereby flex and/or because of their irregular shaping give rise to unevenly sized interspaces in the stack of separation discs. Accordingly, in certain parts of the interspaces (e.g. far away from a spacing member), the mutually adjacent separation discs may be completely compressed against each other to leave no interspaces at all. In other parts of the interspaces (e.g. in the vicinity of a spacing member) the separation discs will not flex much and accordingly provide an adequate height.
A high performing disc stack depends however, among other things, on the interspaces being equidistant. Having the same height all over means that the liquid mixture is evenly distributed in the interspaces of the stack. In this way, the interspaces all contribute to the separation of the liquid mixture. This is important for each of the interspaces separately and in relation to each other. Consequently, different sized interspaces along the stack would cause uneven flow distribution, whereby certain interspaces are overloaded with flow, while other interspaces barely receive any flow at all. Such uneven flow distribution will in turn cause a decrease in the separating efficiency of the disc stack. This problem may also appear in each of the individual interspaces, wherein compressed parts barely receive any flow at all and consequently do not contribute to separating the liquid mixture.